The present invention relates to radial cooled turbine blades and particularly to a system for cooling aft outer span portions of the blades adjacent the trailing edges thereof.
In turbine blade designs which require limited cooling, one approach has been to form generally radially extending drilled holes through each solid cast turbine blade. Typically, compressor discharge air, serving as the cooling medium, is supplied to a plenum at the root of each blade. The cooling air then flows from the plenum generally radially outwardly along the airfoil section of the blade body through straight drilled holes toward the blade tip to cool the blade. The spent cooling air exits the blade tip into the hot gaspath. Because of the airfoil shape of the blade, however, aft portions of the blade cannot be as effectively cooled as forward portions of the blade due to the smaller diameter hole size in the aft portion of the blade necessary to maintain adequate blade wall thickness. That is, the diameters of the aft cooling passages are smaller than the diameters of the more forward cooling passages so that the blade does not encounter structural failure along its aft portions. With these reduced heat transfer surfaces, the cooling medium is rapidly heated. This rapid heat pickup reduces the capacity of the cooling medium to cool the radially outer spans of the aft portions of the blade. This results in inadequate cooling of those aft outer span portions. Consequently, there is a need for a cooling system which will adequately cool the outer aft portions of a cast turbine blade without adverse affect on the structural strength of the blade.
In accordance with a preferred embodiment of the present invention, there is provided a cooling system for a turbine blade wherein cooling air from the adequately cooled forward cooling passages is diverted to one or more of the smaller diameter cooling passages in the outer aft portion of the blade to supplement the flow of cooling air in those latter portions. To accomplish the foregoing, one or more refresher cooling passageways are provided between forward and aft cooling passages at about the midspan portion of the blade. The cooler diverted cooling air supplements the heated cooling air in the aft cooling passages adjacent the aft outer span of the blade.
Particularly, a refresher passageway may extend from a forward cooling passage to a next-adjacent aft cooling passage of smaller diameter. A portion of the cooling air from the large-diameter cooling passage flows through the refresher passageway and combines with the cooling air flowing from the root through the smaller-diameter cooling passage. The refresher passageway communicates with the small-diameter cooling passage at a radial location along the latter passage to effectively cool its aft outer span portion. That is, the inlet to the refresher passageway from the larger-diameter cooling passage is located about midspan of the blade. The outlet from the refresher cooling passageway to the smaller-diameter cooling passage is located radially outwardly of the inlet. This facilitates a natural pumping action of the air from the root, through the cooling passages and refresher passageways to the tips, where the cooling air flows through tip openings into the hot gaspath. It will be appreciated that as the turbine blades rotate, centrifugal forces pump the air radially outwardly. Additionally, the flow of cooling medium from the blade roots to their tips through the cooling passages and refresher passageways is facilitated because high pressure compressor discharge air at the blade roots delivers air to those passages and passageways for exit to a lower pressure region at the tips of the blades. Thus, the cooler air flowing from the refresher passageways into the adjacent smaller-diameter cooling passages refreshes the latter with cooler air affording improved cooling effectiveness. That is, the temperature of the combined flow radially outwardly of the refresher passageways is lower than the temperature of air otherwise flowing through aft cooling passages at those locations without the diverted or supplement flow. As a consequence, aft outer portions of the turbine blades are effectively cooled.
In a preferred embodiment according to the present invention, there is provided a cooling system for a turbine blade comprising a turbine blade body having root and tip portions and a generally airfoil-shaped portion therebetween, a plurality of discrete cooling passages extending from the root portion along the airfoil-shaped portion and to the tip portion for flowing a cooling medium generally radially outwardly along the blade, the passages opening through the tip portion, the passages being spaced one from the other from a forward leading edge of the blade body to an aft trailing edge of the body, at least one passageway interconnecting one of the cooling passages and another of the cooling passages and located in the airfoil-shaped portion between the tip and root portions of the body, one passage and one passageway being located forwardly of another passage, the one passageway having an inlet in communication with one passage and an outlet in communication with another passage, the inlet being disposed along the one passage radially inwardly of the radial location of the outlet along another passage whereby cooling medium flows from one passage through one passageway and into radially outer portions of another passage.
In a further preferred embodiment according to the present invention, there is provided a cooling system for a turbine blade comprising a turbine blade body having root and tip portions and a generally airfoil-shaped portion therebetween, a plurality of discrete cooling passages extending from the root portion along the airfoil-shaped portion and to the tip portion for flowing a cooling medium generally radially outwardly along the blade, the passages opening through the tip portion, the passages being spaced one from the other from a forward leading edge of the body to an aft trailing edge of the body and means for diverting a portion of the cooling medium from one cooling passage for flow to a radial outer portion of another cooling passage aft of the one passage to/supplement the flow of cooling medium through the radial outer portion of another cooling passage.